COHERENT PHONON-PLASMON MODES IN GAAS-ALXGA1-XAS HETEROSTRUCTURES

Time-resolved transient Kerr reflectivity was used to study coherent p honon-plasmon excitations in GaAs:AlxGa1-xAs heterostructures containi ng a two-dimensional electron gas. Ultrafast laser pulses of 65 fs dur ation were used in a balanced optical bridge to observe the oscillator y component of the reflectivity changes directly. This was found to be strongly dependent on the excitation photon energy, showing a crossov er in the spectral amplitude of the longitudinal and transverse-optic- phonon frequencies as the excitation energy was varied. The energy dep endence reflects variations in the phonon-plasmon coupling with depth, and hence local electron density, which is in very good agreement wit h recent predictions.